They spew forth from automobiles and factories, waft up into the air from livestock farms and even come from the ground and other natural sources. Common air pollutants are found all around us, and they can cause severe health effects as well as environmental damage.

Air pollutants are found in the form of solid particles, liquid droplets or gases, and many of them are created by human activity. According to a recent report by the American Lung Association, State of the Air 2011, toxic air pollution hovers above almost every major city, and remains a real threat to the health of the American public despite strong progress in the past few decades. More than half of all Americans live in areas with dangerous levels of air pollution.

Smog in Los AngelesThe U.S. Environmental Protection Agency (EPA) has named the six most common air pollutants, which are found all over the United States. These pollutants are ozone, particulate matter, carbon monoxide, nitrogen oxides, sulfur dioxide and lead. Of these six, ozone and particulate matter are the most prevalent and the most harmful to human health and the environment.

Here's the list:

Comprised of three oxygen atoms, ozone is created at ground level by a chemical reaction between oxides of nitrogen (NOx) and volatile organic compounds (VOCs) in the presence of sunlight. Depending on its location in the atmosphere, ozone can be 'good' or 'bad.'

'Good' ozone occurs naturally in the stratosphere, 10 to 30 miles above the surface of the earth and it forms a layer that protects life on earth from the powerful rays of the sun. 'Bad' ozone contains motor vehicle exhaust, industrial emissions, chemical solvents and other hazardous substances, forming the bulk of the clouds of smog that form over many urban areas.

Particulate Matter
Otherwise known as 'soot,' particulate matter is a mixture of both tiny solid particles and liquid droplets made up of any number of potentially hazardous components including acids, organic chemicals and toxic metals as well as soil or dust particles. Particulate matter falls into two categories:

  • 'Inhalable coarse particles' are between 2.5 micrometers and 10 micrometers in diameter. They are found near roadways and dusty industries.
  • 'Fine particles' are 2.5 micrometers or smaller and are emitted during forest fires, and can also form when gases emitted by power plants, factories and automobiles react in the air. Both categories can pass through the throat and nose and enter the lungs.
Carbon Monoxide
Carbon monoxide (CO) is an odorless, colorless, non-irritating but very poisonous gas emitted from combustion processes that can reduce oxygen delivery into the body's tissues and organs, including the heart and brain, when inhaled. At high levels, carbon monoxide can cause death. Most carbon monoxide emissions in ambient air come from mobile sources.

Emissions from a power plantNitrogen Oxides
The group of highly reactive gases known as nitrogen oxides (NOx) are emitted by high-temperature combustion and often appear as a brown dome of haze over cities. Of the group of nitrogen oxides, which also includes nitrous acid and nitric acid, nitrogen dioxide (NO2) is of the greatest concern to the EPA. It contributes to the formation of ground-level ozone and fine particle pollution, and is linked to adverse effects on the human respiratory system.

Sulfur Dioxide
Part of a group known as 'sulfur oxides' (SOx), sulfur dioxide (SO2) is a chemical compound produced by volcanic eruptions and industrial processes. The largest sources of sulfur dioxide emissions are from fossil fuel combustion at power plants. In the presence of a catalyst like nitrogen dioxide, sulfur dioxide can oxidize into acid rain. It, too, is linked to many adverse health effects on the respiratory system.

Lead is a toxic heavy metal, found naturally in the environment. It's a common pollutant in manufactured products. Motor vehicles and industries are the largest source of lead emissions, and while these emissions dramatically dropped by 95 percent between 1980 and 1999 thanks to regulatory efforts, they are still a concern. The highest levels of lead in the air are currently found near lead smelters. Lead can affect the nervous system, kidney function, immune system, reproductive and development systems and the cardiovascular system.

Know of other common air pollutants? Leave us a note in the comments below.

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